1. Field of the Invention
The present invention relates to an apparatus for polishing wafers and, particularly, to an apparatus for polishing semiconductor wafers relying upon a chemical-mechanical polishing (CMP) method.
2. Description of the Related Art
In recent years, ICs have been finely machined and IC patterns have been formed in a multiplicity of layers. Some degree of roughness is inevitably formed in the surfaces of the layers on which the patterns are formed. According to the prior art, the pattern of the next layer has been formed without any treatment. As the number of the layers increases and as the widths of the lines and holes decrease, however, it becomes difficult to form favorable patterns and defects occur easily. Therefore, it has been attempted to form the pattern of the next layer after the surface of the layer on which the pattern is formed is flattened by polishing. The wafer-polishing apparatus (CMP apparatus) based on the CMP method is employed to polish the wafer in the step of forming the IC patterns.
There has been widely employed a wafer-polishing apparatus comprising a disk-like polishing table having a polishing pad stuck on the surface thereof, a plurality of wafer-holder heads that hold the surfaces on one side of the wafers to be polished and bring the other surfaces of the wafers into contact with the polishing pad, and a holder head drive mechanism for turning the wafer-holder heads relative to the polishing stool, and wherein a slurry which is a polishing agent is supplied between the polishing pad and the wafers to polish the wafers.
There have been proposed various wafer-holder mechanisms for the wafer-holder heads, and some of them are the devices of the floating type as disclosed in Japanese Unexamined Patent Publications (Kokai) No. 6-79618 and No. 10-175161. According to these devices, the wafer is intimately held by being adsorbed by the carrier constituting a portion of the holder head, and the carrier is pushed to push the wafer onto the polishing pad to polish the wafer. An improved holder head in the wafer-polishing apparatus has also been known as taught in Japanese Unexamined Patent Publication (Kokai) No. 8-229808 and in Japanese Patent Application No. 10-92030 filed by the present applicant as schematically illustrated in FIG. 2.
As shown in FIG. 2, this holder head 10 comprises a carrier 20 that is in contact with the back surface of a wafer 2 that is to be polished to push it onto a polishing pad 3, a retainer ring 30 formed in a cylindrical shape to surround the carrier 20 and to push the polishing surface of the polishing pad 3 around the periphery of the wafer 2, a head body 40 provided over the carrier 20 and the retainer ring 30, a drive unit 41 for rotating the head body 40, an adjusting unit 50 (corresponds to a first pushing means) provided between the head body 40 and the carrier 20 to adjust the polishing pressure given to the carrier 20, and an adjusting unit 60 (corresponds to a second pushing means) provided between the head body 40 and the retainer ring 30 and gives a pushing force to the retainer ring 30 that pushes the polishing pad 3 and adjusts the pushing force. These adjusting units 50, 60 are formed of, for example, air bags. The air bag for the carrier at a central portion under the lower surface of the head body 40 is independent from the air bag for the retainer ring along the outer periphery thereof. These air bags separately work to adjust the pushing forces of the carrier 20 and the retainer ring 30.
In general, the pushing forces of the carrier and the retainer ring are equal per a unit area or are slightly different. The retainer ring has a width of usually from 10 to 20 mm and has an area which is about one-fourth to one-half the area of the wafer. In the conventional device as shown in FIG. 2, the air bag for the retainer ring is located along the outer periphery under the lower surface of the head body. Therefore, the area for receiving the pressure increases unless the width A of the air bag is narrowed, making it difficult to finely control the pushing force; i.e., the air pressure must be finely controlled making it difficult to precisely control the-pushing force.
Besides, when the rubber width A of the air bag is narrow, the rubber is not elongated to a sufficient degree making it difficult to control the pushing force.
The present invention was accomplished in view of the above-mentioned problems, and its object is to provide an apparatus for polishing wafers equipped with air bags which enable the rubber width A to be broadened so that the elasticity of a rubber sheet of the air bags for pushing a carrier and a retainer ring can be sufficiently utilized and that the air pressure in the air bags needs not be finely controlled.
As a means for solving the above-mentioned problems, the present invention provides an apparatus for polishing wafers described in the claims.
In the apparatus for polishing wafers according to an embodiment of the present invention, the force for pushing the carrier is obtained from a first pushing means along the outer periphery on the lower surface of the head body and the force for pushing the retainer ring is obtained from a second pushing means at a central portion under the lower surface of the head body, maintaining a pressure-receiving area of the carrier that requires a large force and arranging the second pushing means for the retainer ring without causing its width A to be reduced.
In the apparatus for polishing wafers according to another embodiment of the present invention, the first and second pushing means comprise air bags made of a rubber sheet, enabling the elasticity of the rubber sheet to be sufficiently utilized, and the rubber width A to be increased, so that the air pressure can be controlled even with a small force instead of a very fine pressure control.
In the apparatus for polishing wafers according to a further embodiment of the present invention, the first and second pushing means are applied to the apparatus in which the carrier is equipped with an air-blow member.
The present invention may be more fully understood from the description of preferred embodiments of the invention set forth below, together with the accompanying drawings.